Methods of joining structures and joints formed thereby

ABSTRACT

A product including a first structure. A joint element is attached to the first structure. The joint element has a base and at least two legs extending from the base, the legs defining a slot having opposing sides and a bottom. A second structure is positioned in the slot. An adhesive is disposed in the slot joining the second structure to the joint element. A cavity is formed between the bottom of the slot and the second structure. The cavity being substantially free of adhesive.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a division of prior U.S. application Ser. No.11/137,222, filed May 25, 2005, which application is incorporated hereinby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under a grant from theU.S. Air Force (F33615-48-3-5103). The U.S. government has certainrights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to a method of joining structures, andspecifically a method of joining two structures using an adhesivelybonded joint.

A common method for connecting structures is by mechanical fastening.Mechanical fastening includes connecting a first structure 10 to asecond structure 12 directly using mechanical fasteners 14, as shown inFIG. 1, or connecting the first structure 10 to a second structure 16indirectly by way of one or more intermediate components 18 andmechanical fasteners 14, as shown in FIG. 2. Although mechanicalfastening is often acceptable in terms of cost, weight, complexity andstrength, fastening methods having still lower costs, complexity, andweight, without compromising strength, are always desired.

One substitute for mechanical fastening is adhesive bonding. FIG. 3shows a traditional adhesive joint, generally designated by referencenumber 30, including a double lap joint element 32 (also referred to asa clevis or “pi” joint element) having a slot 34. The joint element 32is connected to a first structure 36 and bonded to the second structure38 by an adhesive 40. A plug or nugget 42 of adhesive fills a bottom 44of the slot 34. Fastening by adhesive bonding is generally less costlythan mechanical fastening because it does not require formation of holesusually required for mechanical fastening.

Although adhesively bonded joints are not necessarily lighter thanmechanically fastened joints, in many cases they are. This differenceoften results from the difference in materials required to form thejoints. Another reason for weight difference results from differences inthe structures being joined. Allowable structural dimensions oftendepend on the type of joint used. For example, although strengthrequirements of a first structure having holes for mechanical fasteningmay mandate a certain thickness of the first structure, the samerequirements may allow a smaller thickness when no holes are present.

Although adhesive bonding is generally simpler and less costly comparedto conventional mechanical fastening, loss of structural integrity hasbeen linked to the presence of nuggets 42 in the bottom 44 of the slot34. Specifically, most adhesives 40 experience some degree of relativecontraction with respect to the joint element 32 and second structure 38during curing. Also, because adhesives 40 generally have a coefficientof thermal expansion greater than that of the joint element 32 andsecond structure 38, the adhesive expands and contracts with respect tothe joint element and second structure during thermal cycling (exposureto relatively high and relatively low temperatures).

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a method of joining a first structureto a second structure using a joint element. The joint element has abase and at least two legs extending from the base and the legs define aslot having opposing sides and a bottom. The method comprises attachingthe joint element to the first structure and inserting the secondstructure into the slot. The method further includes injecting anadhesive into the slot. The method also includes ensuring a cavity isformed between the bottom of the slot and the second structure and thecavity is substantially free of adhesive.

In another aspect, the present invention includes a product ofmanufacture comprising a first structure and a joint element attached tothe first structure. The joint element has a base and at least two legsextending from the base and the legs define a slot between havingopposing sides and a bottom. The product further comprises a secondstructure positioned in the slot and an adhesive disposed in the slotjoining the second structure to the joint element. The product alsocomprises a cavity formed between the bottom of the slot and the secondstructure, said cavity being substantially free of adhesive.

Other aspects of the present invention will be in part apparent and inpart pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional elevation of a mechanical joint according tothe prior art.

FIG. 2 is a cross-sectional elevation of another mechanical jointaccording to the prior art.

FIG. 3 is a cross-sectional elevation of an adhesively bonded jointaccording to the prior art.

FIG. 4 is a cross-sectional elevation of a first embodiment of anadhesively bonded joint according to the present invention.

FIGS. 5A-5F are cross-sectional elevations showing a method of makingthe first embodiment of an adhesively bonded joint according to thepresent invention.

FIGS. 6A-6E are cross-sectional elevations showing a method of making asecond embodiment of an adhesively bonded joint according to the presentinvention.

FIGS. 7A-7D are cross-sectional elevations showing a method of making athird embodiment of an adhesively bonded joint according to the presentinvention.

FIGS. 8A-8E are cross-sectional elevations showing another method ofmaking the first embodiment of an adhesively bonded joint according tothe present invention.

FIG. 9 is a cross-sectional elevation of a fourth embodiment of anadhesively bonded joint according to the present invention.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of joining two or morestructures, and more particularly to a method of joining two structuresusing an adhesively bonded joint having a cavity that is substantiallyfree of adhesive. Referring now to the figures, and more particularly toFIG. 4, a joint according to a first embodiment of the present inventionis designated in its entirety by reference number 50. The joint 50connects a first structure 52 to a second structure 54 using a jointelement 56 and an adhesive 58. The second structure 54 has faces 60 anda lower surface 62. The joint element 56 has a base 64 and two legs 66extending from the base. The legs 66 define a slot 68 having opposingsides 70 and a bottom 72.

The bottom 72 of the slot 68, the sides 70 of the slot, the adhesive 58,and the lower surface 62 of the second structure 54 define a cavity 74that is substantially free of adhesive 58. The cavity 74 can be keptfree of adhesive 58 in a variety of ways. The ways of keeping the cavity74 free of adhesive 58 can be separated into two categories: impedingflow of the adhesive into the cavity during injection of the adhesiveinto the slot 68 and removing adhesive from the cavity during and/orafter injection of the adhesive into the slot. The adhesive 58 may beinjected into the slot 68 before, during, and/or after the secondstructure 54 is inserted into the slot.

A variety of materials can be used as the adhesive 58 and the jointelement 56, as will be appreciated by those skilled in the art. Althoughother adhesives 58 may be used without departing from the scope of thepresent invention, in one embodiment the adhesive is EA 9394, availablefrom Loctite of Rocky Hill, Conn. In one embodiment, the adhesive 58used has a coefficient of thermal expansion of between about20.times.10.sup.−6 in./in.-.degree. F. and about 45.times.10.sup.−6in./in.-.degree. F. Although the joint element 56 may comprise othermaterials without departing from the scope of the present invention, inone embodiment the joint element comprises steel. Other possible jointelement 56 materials include other metals, ceramics, cement, andcomposites, such as a fiberglass epoxy. The joint element 56 may alsoinclude a laminate.

FIGS. 5A-5F show an embodiment in which the adhesive 58 is injected intothe slot 68 of the joint element 56 before the second structure 54 isintroduced into the slot. The adhesive 58 may be injected into the slot68 through a top opening 80 of the slot, as shown in FIG. 5A. Adhesive58 may also be injected into the slot 68 adjacent the bottom 72 of theslot. For example, adhesive 58 can be injected into the slot 68 by wayof a feed hole (not shown) formed in one or more of the joint elementlegs 66. After the joint element 56 is filled at least part way withadhesive 58, the second structure 54 is plunged into the slot 68, asshown in FIGS. 5B and 5C. Any excess adhesive 58 overflowing above thetop opening 80 of the slot 68 as a result of being displaced by thesecond structure 54 may be wiped away or otherwise removed. The secondstructure 54 is spaced from the bottom 72 of the slot 68 by apredetermined distance 82. Presence of the desired distance 82 can beensured in a variety of ways. In one embodiment, presence of thepredetermined distance 82 is ensured by fastening a second structure 84to a joint element 86. For example, a fastener 88 can be placed throughan index hole 90 in the joint element 86 and an index hole 92 in thesecond structure 84, as shown in FIG. 5D. In another embodiment (notshown), index holes in the legs of the joint element are not required.For example, it is envisioned that the fastener 88 may be positionedwithin the second structure 84 so the fastener rests on a top 94 of thejoint legs 66 when the second structure is in the desired position. Asanother example, it is envisioned that the second structure 54 mayinclude one or more protrusions (not shown) that sit on the top 94 ofthe joint legs 66 when the second structure is lowered into place. Inone embodiment, a machine 96 lowers the second structure 54 to a desiredposition and suspends it there until the adhesive 58 cures sufficientlyto hold the second structure in the desired position, as shown in FIG.5E. In this embodiment, no index holes 90, 92 are needed.

After the second structure 54 has been positioned as desired, theadhesive 58 positioned within the cavity 74 is removed so the cavity issubstantially free of adhesive, as shown in FIG. 5F. The adhesive may beremoved in a variety of ways. For example, the adhesive 58 may bedrilled or suctioned out. In one embodiment a brush (not shown) ispushed through the cavity while or soon after the adhesive 58 isinjected. The drilling, suctioning, and brushing may be performed beforethe adhesive 58 is fully cured, which is especially helpful when theadhesive has a long curing time. As will be appreciated by those skilledin the art, each removal method may require the presence of a vent (notshown) to allow air to enter or leave the cavity 74 to avoid a vacuum orhigh pressure condition, as the case may be. For example, a vent caninclude a small hole (not shown) drilled through one of the jointelement legs 66.

In one embodiment, the second structure 54 is inserted into the slot 68of the joint element 56 before the adhesive 58 is injected into theslot. The second structure 54 may be positioned in the slot 68 in avariety of ways as described above. For example, the second structure 54can be positioned using a fastener and index holes or a machine capableof precision positioning. After the second structure 54 is positioned asdesired, the adhesive 58 is injected into the slot 68. The adhesive 58may be injected into the slot 68 in a variety of ways as describedabove. For example, the adhesive 58 can be injected through a topopening 80 of the slot 68 or through a hole (not shown) in the jointelement leg 66. After or while the adhesive 58 is injected, the adhesiveis removed from the cavity 74 of the slot 68. The adhesive 58 may beremoved from the cavity 74 in a variety of ways as described above. Forexample, the adhesive 58 can be drilled, vacuumed, or brushed from thecavity 74.

In another embodiment, the cavity 74 is kept substantially free ofadhesive 58 by positioning a core 100 in the cavity before the injectingstep so adhesive cannot be injected there. In one embodiment, the core100 is a foam that is placed or injected into the slot 68 of a joint102, as shown in FIG. 6A. In one embodiment, the foam core 100 isinjected in a precise amount that will allow the second structure 54 tobe spaced from the bottom 72 of the slot 68 by the predetermineddistance 82 when placed on the foam core. Having the core 100 positionedin the cavity 74 has the additional benefit of creating a seal againstmoisture entering the cavity. Although other foams may be used as thecore 100 without departing from the scope of the present invention, inone embodiment the foam is polysulfide foam having a low modulus ofelasticity. A modulus below about 0.06.times.10.sup.6 psi is consideredlow, compared to the moduli of materials such as aluminum, which has amodulus of about 10.times.10.sup.6 psi, steel, which has a modulus ofabout 30.times.10.sup.6 psi, and carbon fiber reinforced plastic, whichhas a modulus of about 7.times.10.sup.6 psi. It is important to use alow modulus foam to avoid cracks or spaces in the joint 102 duringthermal cycling. In one embodiment, it is preferred that the core 100 beelectrically conductive. A conductive core will promote transfer ofelectricity through the joint 102 when, for example, the first structure52 or second structure 54 become electrically charged. For example, iflightening strikes the first structure 52, it may be beneficial to havethe charge induced by the lightening transferred through to the secondstructure 54 by way of a designed path (i.e., a path including thecore). Otherwise, the charge may take other paths, such as sparkingacross a space, which may cause increased damage and possibly a fire orexplosion if near fuel. Alternatively, it is envisioned that the core100 may be a non-conducting material to electrically insulate the firststructure 52 from the second structure 54.

After the foam core 100 is positioned in the cavity 74, the secondstructure 54 and adhesive 58 are placed in the slot 68. The secondstructure 54 and adhesive 58 may be placed in the slot 68 in a varietyof ways, including those described above. For example, the adhesive 58may be injected into the slot 68 first, followed by insertion of thesecond structure 54, as shown in FIG. 6B. As another way of placing thesecond structure 54 and adhesive 58 in the slot 68, the second structure54 may be positioned in the slot first, followed by injection of theadhesive 58, as shown in FIG. 6C. The foam core 100 keeps the adhesive58 from entering the cavity 74. In one embodiment, the adhesive 58 fillssubstantially all of the slot 68 not occupied by the second structure 54except the cavity 74. The depth of the slot 68 to which the secondstructure 54 is inserted can be controlled in a variety of ways, such asthose already described. In the embodiments shown in FIGS. 6B and 6C, aplacement machine 96 is used to precisely place the second structure 54in the desired location and suspend it in place (as shown in FIG. 6D)until the adhesive 58 can fully support the second structure, as shownin FIG. 6E. In another embodiment, it is envisioned that the secondstructure 54 may be positioned in the slot 68 by placing a predeterminedamount of foam in the cavity 74 so the second structure is properlypositioned in the slot when it is placed on a top 104 of the foam when asurface tension of the foam is sufficient to completely support thesecond structure. It is also envisioned that the foam core 100 may beremoved after the adhesive 58 is cured to produce a structure having ahollow cavity 74 (e.g., see FIG. 5F).

FIGS. 7A-7D show another embodiment that ensures the cavity 74 of ajoint 110 is remains substantially free of adhesive 58 by using a core112 that is a flexible tube. The flexible tube core 112 is placed in thecavity 74 before adhesive 58 is injected into the slot 68, as shown inFIG. 7A. Although the flexible tube core 112 may be made of othermaterials without departing from the scope of the present invention, inone embodiment the tube is made of polysulfide. The second structure 54and adhesive 58 may be placed in the slot 68 in a variety of ways, suchas those already described. For example, the adhesive 68 may be injectedinto the slot 68 first, followed by insertion of the second structure 54into the slot, as shown in FIG. 7B. As another way of placing the secondstructure 54 and adhesive 58 in the slot 68, the second structure may bepositioned in the slot first, followed by the injection of adhesive, asshown in FIG. 7C. The flexible tube core 112 keeps the adhesive 58 fromentering the cavity 74. In one embodiment, the adhesive 58 fillssubstantially all of the slot 68 not occupied by the second structure 54except the cavity 74. A predetermined distance 82 between the secondstructure 54 and the bottom 72 of the slot 68 can be ensured in avariety of ways as described above. For example, in one embodiment, thepresence of the distance 82 is ensured by lowering the second structure54 into the desired place using the precision placement machine 96(shown in FIGS. 5E, 6C, and 6D). In another embodiment, the presence ofthe distance 82 is ensured by inserting a fastener through index holesin the joint element and second structure, respectively (e.g., see FIG.5D).

FIGS. 8A-8E show yet another embodiment for ensuring the cavity 74 of ajoint 120 remains substantially free of adhesive 58 by using a core 122that is an inflatable tube. The inflatable tube core 122 changes shapewhen inflated/deflated. Although the inflatable tube core 122 may bemade of other materials without departing from the scope of the presentinvention, in one embodiment the inflatable tube core is made of latexrubber. The inflatable tube core 122, in a deflated state, is positionedin the cavity 74 before the adhesive 58 and the second structure 54 areplaced in the slot 68, as shown in FIG. 8A. Once positioned in thecavity 74, the inflatable tube core 122 is inflated to substantiallyfill the cavity, as shown in FIG. 8B. Next, the adhesive 58 and secondstructure 54 are positioned in the slot 68 so the second structure isspaced from the bottom 72 of the slot 68 by the predetermined distance82, as shown in FIG. 8C. In one embodiment, the second structure 54 ispositioned in the slot 68 before the inflatable tube core 122 isinflated. The inflatable tube core 122 keeps adhesive 58 from enteringthe cavity 74. In one embodiment, the adhesive 58 fills substantiallyall of the slot 68 not occupied by the second structure 54 except thecavity 74. The presence of the predetermined distance 82 between thesecond structure 54 and the bottom 72 of the slot 68 can be ensured in avariety of ways, such as those already described. For example, in oneembodiment, the presence of the predetermined distance 82 is ensured bylowering the second structure 54 into the desired place using aprecision placement machine 96 (shown in FIGS. 5E, 6C, and 6D). Inanother embodiment, the presence of the predetermined distance 82 isensured by inserting a fastener through index holes in the joint elementand second structure, respectively (e.g., see FIG. 5D). After the secondstructure 54 is positioned in the slot 68 as desired and the adhesive 58has cured sufficiently to hold the second member in place withoutsupport from the inflatable tube core 122, the tube core can be deflatedand removed, as shown in FIGS. 8C-8E.

Although the joints 50, 102, 110, 120 shown in FIGS. 4-8 use a doublelap joint as the joint element 56, 86, other types of joint elementshaving legs defining a slot may be used without departing from the scopeof the present invention. For example, in one embodiment a joint 130includes a joint element 132 comprising two L-brackets 134, as shown inFIG. 9. Each L-bracket 134 has a foot 136 and a leg 138 extending fromthe foot 136. The L-brackets 134 are attached to the first structure 52adjacent each other to form the joint element 132. In this embodiment,the base of the joint element 132 is formed by the feet 136 of theL-brackets 134 (i.e., the base of the joint is effectively split andcomprises the feet). The legs 138 of the L-brackets 134 define a slot140 having opposing sides 142 and a bottom 144. The sides 142 of theslot 140, the bottom 144 of the slot, the adhesive 58, and the lowersurface 62 of the second structure 54 define a cavity 146 that issubstantially free of adhesive 58.

Eliminating the adhesive 58 from the cavity 74, 146 of the joint 50,102, 110, 120, 130 according to the present invention does not reducejoint strength because the joint is nearly unloaded in the cavity area.That is, the adhesive 58 formed between the second structure 54, 84 andthe legs 66, 138 of the joint element 56, 86, 132 accommodatessubstantially all of the load experienced between the joint element andthe second structure. Because adhesive 58 is not needed in the cavity74, 146 of the joint 50, 102, 110, 120, 130, the cavity can be made orleft empty or filled with a core 100, 112, 122 made of a material thatwill not expand/contract due to thermal cycling to degrees sufficient tocause spaces or cracks in the joint. Joints 50, 102, 110, 120, 130formed according to the present invention have been found to havesubstantially no unwanted spaces or cracks formed in or adjacent thecavity 74, 146 under various manufacturing and testing conditions,including thermal cycling between about 220.degree. F. and about−65.degree. F.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the”, and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including”, and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

1. A product of manufacture comprising: a first structure; a jointelement attached to the first structure, the joint element having a baseand at least two legs extending from the base, the legs defining a slothaving opposing sides and a bottom; a second structure positioned insaid slot; an adhesive disposed in said slot joining the secondstructure to the joint element; and a cavity formed between the bottomof the slot and the second structure, said cavity being substantiallyfree of adhesive.
 2. A product as set forth in claim 1 wherein saidjoint element includes a double lap joint element.
 3. A product as setforth in claim 1 wherein said joint element includes at least twoL-brackets positioned adjacent each other to form said slot.
 4. Aproduct as set forth in claim 1 wherein the adhesive fills substantiallyall of the slot not occupied by the second structure except said cavity.5. A product as set forth in claim 1 further comprising an electricallyconductive material positioned in said cavity.
 6. A product as set forthin claim 1 further comprising a flexible tube positioned in said cavity.7. A product as set forth in claim 1 further comprising a materialhaving a low modulus of elasticity positioned in said cavity.
 8. Aproduct as set forth in claim 7 wherein said material comprisespolysulfide.